Injection molding device and method

ABSTRACT

A novel method and device for precision injection molding is disclosed providing controlled cooling of a portion of the mold cavity surface during the molding cycle, or several such portions independently, and also, in the preferred embodiment controlled heating of a portion of the mold cavity surface during the molding cycle or several such portions independently, whereby high quality, high precision parts having close dimensional tolerances may be produced. Another aspect of the invention, a volume-controlled variable conductance heat pipe is disclosed, which novel heat pipe comprises housing means forming a sealed chamber, fluid, such as water or ammonia, within the chamber, wicking means and control means for controlling the thermal conductance of the heat pipe comprising means for controlling the volume of fluid in the liquid phase in the chamber. Preferably, the novel injection mold comprises a thin mold face supported at least in part by a foundation means which either incorporates, or is itself, a novel volume controlled variable conductance heat pipe as disclosed.

This invention relates to an injection molding device and method, and toa novel variable conductance heat pipe which may be used in such deviceand method. More specifically, it relates to an injection molding deviceand method which provide controlled differential mold cooling andheating capability.

Injection molding is a process by which some malleable material isforced under pressure into a closed mold. The material solidifies andretains the shape of the mold. Thermal-plastic materials, thermosettingmaterials and ceramic materials can be processed in this way. In atypical injection molding process, a material is melted and injectedinto a mold that is clamped closed. The material freezes in therelatively colder mold and is then ejected.

At the beginning of the molding cycle, material is melted and theninjected into the mold through the sprue bushing, runner, and gate.During injection, the melted material (the melt) is subject to a coolingeffect by contact with the relatively lower temperature surface of themold but is subject to a heating effect due to viscous dissipation inthe melt. If the cooling effect is much greater than the heating effect,the plastic will solidify before the mold is filled, resulting in ashort shot. If the heating effect dominates, the molding cycle can beunnecessarily extended. The melt temperature and injection rate must bechosen so that neither of these problems occurs.

At the end of the injection period the flow in the mold stops, thepressure rises rapidly and the material begins to cool. As the materialcools it shrinks slightly and more material is formed into the cavitydue to the hold pressure acting on the melt. This portion of the moldingcycle is called the hold or the packing stage and continues until thehold pressure is released or until the gate freezes. After the gate hasfrozen, the material continues to cool, which at first causes areduction in pressure and then shrinkage of the material in the cavity.When the part has cooled sufficiently to remain rigid, the mold isopened and pins eject the part, runner, and sprue from the mold.

Conventional injection molding techniques have been found inadequate forproduction of molded parts in certain instances. Typically, for example,high precision parts having close dimensional tolerances often cannot beproduced employing conventional injection molding techniques. Moldedparts, especially those having non-uniform wall thickness, are oftenfound to undergo warping, cracking and other distortions believed to bedue in part to molded-in stresses within the molded part. Suchdeficiencies in current injection molding techniques preclude their usefor production of many precision parts and increase the cost of closetolerance molded items. Reduction or elimination of these problems wouldlead to broader application of injection molding methods and reducecosts, for example, by eliminating secondary operations such as sanding,painting, and drilling that are often necessary to obtain precisionparts. Moreover, waste could be reduced by a sufficiently preciseinjection molding method.

Shrinkage of the melt material after the gate has frozen is asignificant problem. Shrinkage occurs with various molding materials andis usually compensated for by designing the mold over-size. That is, theapproach generally taken to reduce deviations from the desired shape ofthe molded part is to compensate for shrinkage and other distortions byaltering the mold cavity design. Shrinkage can vary, however, forexample, from about 0.7% for PMMA molded at 20,000 psi to about 7% forpolyethylene molded at 5,000 psi. Also, the shrinkage amount oftendepends on several molding conditions, such as injection rate,temperature, and configuration of the molded part, which cannot becalculated exactly. The shrinkage of nylon, for example, depends on thedegree of crystallinity, which in turn depends on injection flowpatterns and cooling rate. Distortions due to shrinkage may show up, forexample, as sink marks (surface depressions), bending, and parts out ofround. Thus, for example, employing current injection moldingtechniques, it is often difficult to hold a roundness dimension closerthan about 0.7%, or a concentricity dimension closer than about 0.5%.Thus, while it is simple in theory to compensate for shrinkage bydesigning the mold over-size, generally the mold designer cannotaccurately predict the final shape of the part and therefore cannotcompletely compensate for these problems. These variations in shrinkageand the consequent, often unpredictable distortions in the shape of themolded part have caused current injection molding techniques to be foundunacceptable when close dimensional tolerances are specified for themolded part.

Another shape distortion problem presented by current injection moldingtechniques is warping, which along with shrinkage, contributes to theoverall problem of precision shape reproduction. Warping is common alongflat thin surfaces of parts and distortion of regular geometric shapesor spacial relationships often occurs in molding. In thick sections orribbed sections, a sink mark is a common distortion problem. Ifaesthetic considerations are important, even very small sink marks canbe a serious problem.

Obtaining satisfactory mechanical, optical, and environmental propertiesfor the molded part can also present significant problems. Molded partsoften exhibit anisotropic mechanical properties, poor impact strengthand poor resistance to solvent cracking. Moreover, the as-moldedcondition of parts often is not stable. Properties and dimensions ofparts can change after molding.

The approach generally employed to reduce the above-described injectionmolding problems is to evaluate a number of trail molding cycles toidentify an acceptable cycle. The evaluation by trail process is used todesign molds, set up molding operations, and to produce parts. Thisoften difficult approach has significant drawbacks which include expenseand production delays. Delays occur due to the iterative nature of theapproach and the time required to make the necessary adjustments at eachstep. Large expense may result from non-productive men and machineduring each redesign period and from the production of scrap parts andwasted materials. Also, the cost of mold modification can be high.Identifying the proper injection molding cycle by trial and error isalso difficult due to the interdependence of the numerous processvariables, such as melt temperature, injection pressure, partconfiguration, among others. Thus, it is often difficult or impossibleto predict the effect of a change on the final characteristics of themolded part.

It is a primary objective of the present invention to provide aninjection molding method and device to produce injection molded partshaving close dimensional tolerances, which reduces the need forevaluation of trial molding cycles to identify an acceptable cycle.

It is also a primary objective of the present invention to provide aninjection molding method and device wherein the shrinkage anddimensional distortions need not be compensated for by alteration of themold cavity design.

It is an object of the present invention to provide an injection moldingmethod and device suitable to produce high precision molded parts havingclose dimensional tolerances, and molded parts having non-uniform wallthickness throughout the part, which undergo little or no warping orcracking or other distortion.

It is an object of the present invention to provide an injection moldingmethod and device for production of molded parts having isotropicmechanical properties, good impact strength and good resistance tosolvent cracking. It is likewise an object to provide such device andmethod suitable for the production of molded parts whose as-moldedcondition is substantially stable over time.

It is an object of the present invention to provide an injection moldingmethod and device wherein little or no shrinkage of the melt materialoccurs after the gate has frozen, and to provide such a method anddevice wherein there is no need to alter the design of the mold cavitydesign to compensate for shrinkage. It is an object to preventdistortions in injection molded parts, otherwise resulting due to theshrinkage, such as sink marks, bending and parts out of round.

It is another object of the present invention to provide an injectionmolding method and device suitable for production of molded parts whichundergo little or no warping, even along flat, thin surfaces of theparts.

According to the present invention, an injection molding device forproduction of high quality, close dimensional tolerance, injectionmolded parts comprises an injection mold having at least one and mostoften two or more mold pieces defining a mold cavity, cooling means forcooling at least a portion of the mold cavity surface and coolingcontrol means for controlling heat flow from at least a portion of themold cavity surface. The cooling of at least one portion of the moldcavity surface is controlled independently of the cooling of otherportion(s) of the mold cavity surface. The cooling control meanspreferably has a sufficiently fast response time, such that the moldingcycle is not significantly extended. The cooling control meanspreferably comprises variable conductance heat pipes and most preferablyvolume controlled variable conductance heat pipes having means forcontrolling the liquid volume and/or total fluid content within the heatpipe(s). The injection molding device also preferably comprises heatingmeans for heating at least a portion of the mold cavity surface.Preferably, at least one mold piece comprises a thin mold face formingthe cavity-side surface thereof and heating means are incorporated intothe thin mold face. In this preferred embodiment, a portion or all ofsuch thin mold face is directly supported in part by the main mold frameand in part by suitable foundation means. The foundation means comprisesa support structure of sufficient compressive stiffness to providedimensional stability to the supported thin mold face during the moldingcycle. The support structure preferably has suitably low thermal inertiasuch that the molding cycle is not significantly extended by longcooling time requirements, and preferably the aforesaid cooling means,such as heat pipes, are incorporated into the support structure. Thecooling control means may be located at a suitably accessible remotelocation. The main mold frame may be constructed, for example, of solidsteel to provide sufficient rigidity. Conventional mold cooling channelsin the main mold frame and other conventional injection mold featuresmay be provided in the known fashion. The main frame may be mounted tothe platens of an injection molding machine in the known fashion.

The operation and advantages of the present invention will be moreapparent from the following drawings and description of illustrative andpreferred embodiments of the present invention.

IN THE DRAWINGS

FIG. 1 is a schematic cut-away drawing of an exemplary injection moldingdevice according to a preferred embodiment of the present invention.

FIG. 2 is a schematic drawing of a volume controlled variableconductance heat pipe according to a preferred embodiment wherein theconductance is varied by fluid content control means.

FIG. 3 is a schematic drawing of a variable conductance heat pipeaccording to a preferred embodiment wherein the conductance is varied byvolume control means comprising a bellow means.

FIG. 4 is a graph showing the relationship between the initial melttemperature and initial melt pressure required to obtain substantiallyzero volume change during molding.

While not intending to be bound by theory, it is helpful to anunderstanding of the present invention to consider a molded part ascomprised of minutely small contiguous elements of molding material.During the conventional molding cycle, post-injection flow of materialoften occurs between different portions of the molded part, i.e., fromone such element to another, which can result in anisotropic molecularorientation, sheering and ultimately distortion of the molded part. Thismay be referred to as "secondary flow". That is, after injection of themolding material into the mold, the temperature drops and the tendencyis for shrinkage to occur in response to the change in temperature. Whenthe density, and therefore the volume of material within a given minuteelement of the molded part is dependent on the temperature of thatmaterial, it can be seen that the average temperature of each suchminute element must remain approximately equal to the averagetemperature of each other element during the molding cycle if there isto be no substantial flow into or out of any one of such minuteelements. At the same time, the pressure in the mold drops and thetendency of the molding material is to expand due to the lower pressure.Therefore, unless the material throughout the molded part issufficiently temperature homogenous and isotropic during the cooling andsolidification of the part, anisotropic molecular orientation within themolded part can result due to sheering or extending the melt just priorto solidification within the mold. By insuring a sufficientlyhomogeneous temperature and isotropic condition, at least on averageover a region of the part, to eliminate secondary flows between regionswithin the molded part, the conditions of substantially homogeneous andisotropic density, stress and molecular orientation can be achieved.

According to the present invention, by controlling the cooling rate ofat least one portion of the molded part independently of other(s) duringthe molding cycle, the average temperature of each region within thepart is kept sufficiently close to the temperature of at least thoseregions proximate to it. This is accomplished by controlling the heatflow from the surface of the mold cavity. Approximate temperatures atpoints within a cooling molded part can be computed numerically usingthe heat conduction equation or by using graphical solutions or othermethods well known to those skilled in the art. Especially with the aidof suitable computing machinery, it will then be well within the skillof the art to identify those portions of a molded part from whichgreater or lesser heat flow will be required to maintain a sufficientlyconsistant temperature throughout the molded part as it cools.

Cooling of the mold cavity surface according to the method and device ofthe present invention varies not only from one portion thereof toanother. Rather, it also varies with time since the desired rate of heattransfer from the melt, or any portion thereof, will generally notremain constant throughout the molding cycle. It may be necessary ordesirable at times during the cooling of the melt to change the rate ofheat absorption from one or more of the independently cooled portions ofthe molded part to maintain the sufficiently consistent temperaturethroughout the molded part during the cooling portion of the moldingcycle. Thus, according to the present invention, the cooling meansprovided to each portion of the mold cavity surface will have variableconductance responsive to cooling control means enabling regulation ofthe rate of heat transfer provided by that cooling means.

Moreover, especially for purposes of economy of time, and consequentlyof cost, it is highly desirable that the control means be capable ofchanging the rate of heat transfer rapidly. Since the rate at whichinjection molded parts can be produced is determined in large part bythe time it takes to cool the melt, and in view of the heating of thecavity surface provided according to one aspect of the presentinvention, the cooling means will most preferably be capable ofsubstantially instantaneous change from near zero heat absorbtion (e.g.,during heating of the mold cavity surface) to the full desired rate orheat absorbtion (e.g., subsequent to injection). Thus, the cooling meansmust have a sufficiently rapid response to the cooling control means.

In addition to controlling the cooling rate of different portions of amolded part to avoid molding defects which arise during the coolingportion of the molding cycle, it is necessary to minimize or eliminatemolding defects such as anisotropic molecular orientation that resultduring the injection of the molding material into the mold. Due to themolecular structure and high viscosity of common injection moldingmaterials, it is often not practical to completely avoid orientationduring injection. According to the present invention it is possible,however, by heating the molded cavity surface, to allow molecularorientation within the melt to relax before it freezes and orientationlocked-in. Thus, heating means are provided for heating the mold cavitysurface or some portion(s) thereof during a suitable portion of themolding cycle. The heating of different portions of the mold cavitysurface may be independently controlled.

Prior efforts to employ heated mold cavity surfaces to produce moldedparts have been inadequate or impractical for at least two reasons. Thefirst is that the heating step fails to avoid distortions in the moldedpart resulting from secondary flow-induced molecular orientation andshearing. The second is the increased molding cycle time. The timenecessary to allow relaxation of the melt, while dependent in part onthe molecular structure of the injection material, varies greatly withtemperature. Since a commercially practical injection molding cycle mustinvolve a short molding cycle time, heating the mold for sufficient timeto provide adequate molecular relaxation and the consequent increase inthe requisite cooling time significantly increased the overall moldingcycle time. According to the present invention however, means forrapidly absorbing and rejecting heat from the molded part are provided,which cooling means is variable over time and thus does not prevent orsubstantially interfere with heating the cavity surface. Rather, thecooling means is employed only during that portion of the molding cyclewhen the temperature of the mold cavity surface is to be reduced. Thus,the molding device of the present invention enables heating of the moldcavity without substantially increasing the molding cycle time. Byproviding variable and independent cooling means for different portionsof the molded part, and by providing heating means for the mold cavitysurface, a synergistic effect is achieved in that high quality injectionmolded parts of close dimensional tolerance, free of substantiallyeither injection-induced or secondary flow-induced distortion can beproduced within a short molding cycle time.

With conventional injection molding techniques, shrinkage of the moldedpart away from the walls of the cavity often occurs. This may beacceptable since the mold cavity could be made oversize by anapproximate compensating amount. This shrinkage may result in slightdistortion of the molded part, however and thus is preferably to beavoided in the production of high quality molded parts according to thepresent invention. Relatively higher mold cavity pressures are used inthe preferred embodiment of the present invention to produce moldedparts that are substantially the same volume as the cavity. Accordingly,the cavity can be designed substantially exactly to size rather thanoversize by some approximate amount and thus molded parts can be held toclose dimensional tolerances.

The cavity pressure necessary to produce moldings that are the samevolume as the cavity are typically quite high, but will vary from onemolding material to another and will vary with the demolding temperatureand pressure, T_(f) and P_(f) respectively. For illustrative purposes,conditions necessary to obtain substantially zero volume change of PMMAmoldings are shown in the following Example.

EXAMPLE I

Referring now to FIG. 4, T_(i) is the injection temperature. The initialmelt pressure P_(i) defines the condition within the cavity, not theinjection pressure. T_(f), the demolding temperature is the averagetemperature throughout the part and is usually significantly higher thanthe surface temperature. Demolding is at atmospheric pressure.

In view of the present disclosure, it is within the skill of the art toselect suitable pressures for use in conjunction with the presentinvention. Thus, for example, mold cavity pressures of from about 5,000to as high as 20,000 psi or more may be required. The injection moldingdevice must be capable of withstanding the high pressure without damage.Since the clamp force required in an injection molding press is theproduct of the cavity pressure and the area of the cavity projected ontothe plane of clamping, the high cavity pressures will increase theclamping requirement of the machine. The high mold cavity pressures isalso a consideration in the design of the support provided to the moldsurface. At the very least a mold must be supported to the extent thatthere will be no permanent deformation of the mold during the moldingcycle. Where, for example, precision parts must often be produced withintolerances tighter than 0.001" but such parts can be manufactured in amold which deflects more than 0.001" during the cycle if at the finalstage of molding the pressure returns to atmospheric and the elasticdeflections return to zero. The requirements on deflection then willoften be much less severe than the tolerance on a molded part.

A preferred embodiment of an injection molding device according to thepresent invention is shown in FIG. 1. An injection molding device foruse in accordance with the present invention, as shown in FIG. 1,provides differential mold cooling, heating of the mold cavity surfaceand high cavity pressures for the production of injection molded partsof high quality having little or no shape distortions.

Injection mold device (1) comprises mold pieces (2) and (2') definingmold caticy (3) which, for exemplary purposes, is shown to havenon-uniform thickness throughout the part. If used in conjunction withconventional injection molding techniques, such configuration wouldpresent problems of shape distortion and molded parts of low quality asdiscussed above.

Mold piece (2) is exemplary of the most preferred mold piece designaccording to the present invention. Mold piece (2) comprises a thin moldface (4) forming at least a portion of the cavity-side surface thereof.As well known in the art, the mold face must be able to maintain thedesired cavity shape, must maintain a high quality surface finish, mustresist wear and must be able to endure the work environment. Moreoverthe thin mold face must be capable of enduring the thermal cycling ofthe injection molding process according to the present invention andmust therefore comprise a material with sufficiently high thermaldiffusivity. There are numerous manufacturing techniques well known tothose skilled in the art suitable for making the thin mold faceaccording to the preferred embodiment of the present invention. Thus,for example, the conventional method of machining a mold cavity may beused. The cost however of manufacturing a thin mold face in the case ofan irregular cavity geometry may be high since machining of both sidesis required to produce a uniform surface thickness which is generallypreferred. Alternatively, a layer of metal may be built up over amandrel according to methods known to the art. One such method, forexample, is to spray atomized liquid metal onto a mandrel. This may bedone by melting the metal, for example with an arc or with anoxygen-acetylene flame, and propelling the metal onto the form with anairjet by methods well known in the art.

Preferably, however, electroforming is used to deposit metal onto aform. Using methods well known to the art, a layer of plating is builtup to a desired thickness. The plating is then separated from the formand is used as a mold to produce the original form. The mechanicalproperties of the deposited metals have been found to be good and highresolution is obtained. While irregular geometries may result invariations in placing thickness, this problem can generally bealleviated according to methods well known in the plating art, such asfor example by using "thiefs" and "shields" to modify current density.

The low thermal inertia or thin mold face, according to the preferredembodiment, enables it to be heated or cooled rapidly, and thus enablesrapid and accurate control of the heating and cooling rate during themolding cycle.

According to the present invention, heating means are provided forheating at least a portion of the mold cavity surface prior to and orduring injection of the melt. Heating of the thin mold face can beaccomplished in a number of ways. For examples, direct convectionheating could be used by introducing a heated fluid into the coolingchannels (13) of the mold to alternate with the cooling fluid whichwould normally flow therein. However, since cooling channels can notnormally be located sufficiently proximate to the surface of a mold, asufficient change in cavity surface temperature may require substantialtime depending upon the geometry of the mold and the cooling channels.Transporting heat to the thin mold face with a device such as a heatpipe or heating the surface by radiation or conduction providealternative heating methods. According to the preferred embodiment ofthe present invention however as shown in FIG. 1, heat is generateddirectly in the thin mold face by incorporation therein of electricalresistance heaters. Any suitable method of incorporating electricalresistance heaters into the thin mold face may be used. According to asuitable method, the thin mold face is formed around the electricalresistance heaters by an electro-forming process. The electro-formingprocess is interrupted after a thin layer has first been deposited, anelectrical resistance element is laid upon the surface and theelectro-forming process is continued to the desired thickness. In thismanner, the electrical resistance heaters are encapsulated in thedeposited metal, preferably nickel. Such a process yields a thin moldface that has an electrical resistance heater in the desired location,and very near the surface of the molding. Preferably, a Kanthalresistance wire, insulated with a thin teflon coating may beincapsulated in an electro-deposited nickel thin mold face. However,other commercially available heaters may be used.

In accordance with the present invention, different portions of the moldcavity surface may be cooled at different, independently controlledrates. The exemplary preferred embodiment, shown in FIG. 1, variableconductance cooling means (6) is provided for at least a portion of themold cavity surface (4). The cooling means (6) comprises cooling controlmeans (7), shown schematically, for controlling the rate at which thatportion of the mold cavity surface is cooled by heat conductance means(8). As noted above, use of mold surface heating means in conjunctionwith mold surface cooling means is more effective where the rate ofcooling is controllable over time as well as position. Therefore,preferably the heat conductance means has a sufficiently fast responseto the cooling control means such that the molding cycle is notsubstantially extended. Preferably, the heat conductance means comprisesvariable conductance heat pipes. That is, according to the preferredembodiment of the present invention, as shown schematically in FIG. 1,one or more portions of the thin mold face is each cooled by anindependently controlled heat conductance means comprising one or morevariable conductance heat pipes.

The heat pipe is a well known device used for transporting heat. Therate of heat flow can be controlled by methods well known to the art,thereby providing variable heat conductance away from the mold cavitysurface. A heat pipe comprises a (generally pure) fluid, for examplewater or ammonia, and a porous wick structure, comprising for example,fine mesh screen, in a sealed chamber. The fluid is in a two-phase,liquid-vapor state during normal operation of the heat pipe. Heat istransferred into the pipe by conduction through the wall at the hot endof the pipe. This region is called the evaporator. The heater causessome of the liquid in the wick to evaporate which results in a slightincrease in pressure at the evaporator. This pressure gradient drivesthe vapor to the cold end of the heat pipe condenser. Heat is rejectedas the vapor condenses. The rejected heat is conducted through the pipewall to the outside of the heat pipe. The condensed liquid collects onthe wick at the condenser and is pumped to the evaporator by capillaryaction within the wick. The return of the fluid to the evaporatorcompletes the cycle for the fluid. The operation and application of theheat pipes is given by Chi, S.W., Heat Pipe Theory and Practice, McGrawHill Book Co., 1976 incorporated herein by reference.

The control action of heat pipes, such as for example, the gas-loadedand excess-liquid type, occurs at the condenser end of the pipe. Thesedevics are controlled by a change of the conditions at the cold end ofthe pipe. If the desired control action is to stop heat transfer at thehot end, the system operates by stopping heat transfer at the cold end.Heat continues to be transferred into the pipe at the evaporator untilthe heat pipe temperature rises to the temperature of the heatingsurface. The delay between control signal and control action makes theuse of this type of device less preferred for mold temperature controlaccording to the present invention.

Vapor-flow-modulated heat pipes respond more rapidly than gas-loadedpipes, but performance depends on reliable operation of throttlingvalves near the mold surface.

An alternative to these types of devices is a volume-controlledvariable-conductance heat pipe. The control action in this type ofdevice occurs at the hot end of the heat pipe.

Since cooling means (6) preferably responds quickly to cooling controlmeans (7), to adequately control heat flux at the hot end of the heatpipe, i.e. at the mold cavity surface, most preferably a volumecontrolled variable-conductance heat pipe according to the followingdescription is employed. The novel volume controlled variableconductance heat pipes of the present invention function by inhibitingthe flow of liquid to the evaporator by decreasing the volume of fluidin the liquid phase in the heat pipe. This results in decreasedcapillary pumping in the wick or "wick dry out". When this occurs,significant amounts of heat can no longer be effectively absorbed by theevaporator. The change in heat conductance may be obtained by the directremoval or addition of fluid to the sealed enclosure. FIG. 2 showsschematically such a volume controlled variable conductance heat pipe.Housing means (20) forms a sealed chamber (21). A fluid (22) which innormal operation would comprise both a liquid and a vapor phase, iscontained within the sealed chamber. The liquid phase of the fluid wouldnormally reside within wicking means (23) and be carried by wickingmeans to the heat absorbing end of the heat pipe, the evaporator. Fuidcontrol means (24) provides control of the amount of fluid (22) in saidsealed chamber (21) by the addition of or withdrawal of a portion offluid (22) to control the rate of heat absorption at the evaporator. Thefluid control means (24) may comprise a fluid reservoir and a reversiblemetering device forming a connection between the fluid reservoir and thesealed enclosure.

The reversible metering device may, for example, comprise a reversiblemetering pump. Alternately, the liquid volume control means may compriseat least one uni-directional valve. Preferably, 2 uni-directional valveswould be used to connect the sealed enclosure to the fluid reservoir.Preferably, the uni-direction valves are metered valves. The valves maybe solanoid controlled or, alternatively pneumatically controlled.

An alternative volume controlled variable conductance heat pipeaccording to the present invention is shown schematically in FIG. 3.Therein, variable volume bellow means (30) are provided to controllablychange the total volume of the sealed enclosure (21) of the heat pipe.The cavity of bellow means (30) is of controllably variable volume andcomprises a portion of the sealed chamber (21). By causing a rapid,preferably almost instantaneous increase in the volume of the sealedenclosure, some portion of the fluid (22) responds to such increase involume by evaporating into the vapor phase. The resultant liquiddepletion in the wick, causes an almost instantaneous decrease in theavailable capillary pumping capability of the heat pipe. Heat transferat the evaporator effectively stops when the capillary pumping of thewicking means (23) ceases to supply liquid to the evaporator.Accordingly, an increase in volume decreases the conductance of the heatpipe nearly instantaneously. It should be understood that any means ormethod of controlling the total volume of the sealed chamber is intendedto be included within the term bellow means.

Since the control action in the volume controlled variable conductanceheat pipe occurs at the evaporator end of the heat pipe, it has a rapidresponse to the cooling control means. That is, sufficiently rapid andaccurate control of the rate of cooling from the affected portion of themold cavity surface is provided by the volume-controlled variableconductance heat pipes of the present invention. It should be noted thatthe operation of a heat pipe will depend on its orientation in a gravityfield.

The performance of the volume controlled variable conductance heat pipedepends on the degree to which capillary pumping action is affected byliquid volume changes made by the control means, that is, on the loss ofcapillary pumping action due to liquid depletion. In the volumecontrolled variable conductance heat pipe of the type shown in FIG. 2,having means for controlling the total amount by weight of the fluidwithin the sealed chamber, the degree of liquid depletion is controlleddirectly. In the volume controlled variable conductance heat pipe of thetype shown in FIG. 3, having a variable volume bellow means, the degreeof liquid depletion may depend on either the rapidness of the volumechange or the amount of the change or both. In both types of volumecontrolled variable conductance heat pipes, conduction is controlled bycausing a liquid depletion at the hot end of the heat pipe, i.e. at theevaporation. Because heat conductance is controlled at the evaporatorrather than at the condenser, the heat absorption by the evaporation ofthe volume controlled variable conductance heat pipe of the presentinvention responds rapidly to the control means.

In this regard, it is important to note, however, that the dynamicperformance of the volume-controlled heat pipe is quite different fromthe equilibrium, isothermal performance. Consider first an isothermalexpansion of the volume of the sealed chamber. The volume of liquiddriven into the vapor phase during an isothermal expansion depends onthe fluid and on the operating conditions. For example, at 300° F., a370 cm³ volume change is required to drive 1 cm³ of water to the vaporphase. Only 9% of that volume change is necessary if the water is at500° F. and only 2.8% is needed if the water is at 600° F. Note,however, that the benefits of operating volume controlled heat pipesnear the critical point, are offset in some applications by the reducedheat transport capability at elevated temperatures. For example, a waterheat pipe at 500° F. has only 70% of the capability of a heat pipe at300° F.

If the heat pipe volume is changed very rapidly, the conditions insidethe pipe are more nearly adiabatic than isothermal. In such a case, theliquid volume decreases only slightly at first and the pressure andtemperature drop. Heat conducted into the device causes more of theliquid to be vaporized until the device reaches the steady state. Thatis, during the change, the heat transfer rate at the hot end increasesbriefly and then decreases. Consider for example a heat pipe that ismaintained at 500° F. The heat pipe volume is suddenly increased. Theliquid volume decreases by 2.5% and the temperature drops to 489° F.Heat continues to be transferred into the pipe for a brief time untilthe temperature rises to 500° F. At this new condition the liquid volumeis 90% of the original liquid volume and the overall conductance of thedevice has been reduced approximately 75%.

While the temporary increase in heat transfer was contrary to theultimate result, the changes occur rapidly at the hot end of the pipesso that it remains possible to use the volume controlled variableconductance heat pipe to accurately profile the heat transfer rate atthe evaporator. In fact, this aspect of the novel heat pipes of thepresent invention can be advantageously relied upon to accuratelyprofile heating and cooling during the molding cycle. By changing thevolume at the appropriate time during the molding cycle, the temporarilyincreased cooling effect and subsequent rapid reversal can be made tocoincide with those portions of the molding cycle during which coolingand then heating, (or at least decreased cooling), respectively, isneeded. In this way, increased accuracy can be achieved in profiling thecooling and heating of the mold during the molding cycle. Similarly, theopposite effect, a brief decreased cooling and subsequent rapid reversalto increased cooling when the heat pipe volume is decreased can beadvantageously employed to accurately control the cooling of the moldduring the molding cycle.

An important advantage of using volume-controlled variable conductanceheat pipes for mold cooling, according to the present invention is thatthe heat transfer at the evaporator can be controlled directly.Evaporator control allows faster and more accurate control of thetemperature and the heat transfer rate at the mold cavity surface. Thus,the dynamic response characteristics of these devices can be utilized toprofile the cooling rate over time. However, the volume controlledvariable conductance heat pipe of the present invention may be used notonly in the presently disclosed injection molding invention, but alsowhere a conventional heat pipe would otherwise be used or in anyapplication where heat transfer is to be controlled. Moreover, it isuseful in certain applications requiring more rapid or more precisecontrol of heat conductance than is provided by a conventional heatpipe. It is envisioned for example, that certain embodiments of thepresent disclosed injection molding method and device would present suchan application. Thus, the volume controlled variable conductance heatpipes of the present invention is a significant advance in the artindependent of its application in the injection molding device andmethod.

Suitable wicking means include the materials known to the art, forexample, one or more layers of 250 pores/inch mesh wrapped phosphorbronze screen. Suitable housing means to form the sealed chamber maycomprise any suitable material such as those well known to the art andany suitable form, adapted to the particular application. Cylindricalhalf inch O.D. cooper heat pipe is exemplary. The heat pipe materialsand fluid should be selected to assure sufficient compatability. Thus,for example, a sufficiently compatible heat pipe may comprise housingmeans of copper pipe, wicking means of copper screen and water as thefluid. Alternately, stainless steel pipe, fine stainless steel screenand water of ammonia may be used.

Thermocouples or other suitable temperature monitoring means may beincluded in the heat pipe, for example, at the evaporator end to monitorperformance of the heat pipe or other purposes.

Having shown and described the most preferred heat conductance means(8), the description of the preferred embodiment of an exemplaryinjector molding device according to the present invention can becontinued.

FIG. 1 shows that portions of the thin mold face (4) may be supported bymain mold frame (9), while other portions are supported by a foundationmeans (10). The foundation means comprises a support structure ofsufficient compressive stiffness to provide dimensional stability to thesupported thin mold face during the molding cycle. In addition tosupporting the mold face, the support structure in the preferredembodiment provides access to the thin mold face for the cooling means.The support structure must either be thermally isolated from the thinmold face or preferably have a low thermal inertia such that it does notsubstantially increase the time required for heating and cooling thethin mold face. In the preferred embodiment, the mold is cooled with thenovel volume controlled variable conductance heat pipes of the presentinvention. The heat pipes preferably are built into the foundation meansto transfer heat from the thin mold face to the main mold frame (9)according to such preferred embodiment, for example, a foundation meansmay comprise a suitable low thermal inertia rigid epoxy material,suitable heat pipe(s), for example cylindrical heat pipe(s) according tothe present invention, are positioned within and through the epoxy. Theevaporator is placed at the thin mold face and the heat pipe(s) extendedthrough the foundation means to the main mold frame. The cooling controlmeans can make connection to the heat pipe(s) within the foundationmeans. The rapid response of the novel heat pipes and the low thermalinertia of the thin mold face together provide more rapid and accuratecontrol of the heating and cooling rate during the molding cycle thancould be achieved, for example, with a conventional solid steel moldingpiece.

Alternate to incorporating heat pipes into the support structure, thesupport structure can be used to function itself as the presentlydisclosed novel heat pipe. Thus, for example, the foundation means maycomprise a sealed chamber and one or more parallel columns, for examplesolid stainless steel columns, extending from the thin mold face to themain mold frame. The columns may be wrappered with fine stainless steelscreen and the sealed chamber is charged with a suitable fluid, forexample water. Suitable cooling rate control means may comprise bellowmeans outside the main mold frame having a sealed connection to thefoundation means. In such an embodiment of the injection molding deviceof the present invention, the foundation means itself comprises a novelheat pipe according to the present invention.

Control means for the heating means and the cooling control means may besuitably located at any convenient and accessible position. For example,the heat conductance means can comprise the novel volume controlledvariable conductance heat pipes of the present invention and the volumecontrol means may be located remote of the main mold frame.

Where volume controlled variable conductance heat pipes are incorporatedinto the support structure of the foundation means, that is, where theheat pipes extend from the frame, a most preferred embodiment as shownin FIG. 1 provides small channel(s) (11) in the main mold frame forminga portion of the heat pipe(s) and operates as at least a portion of thecondenser thereof. In this manner heat transfer to the main mold frameand ultimately to the cooling channels is improved.

It should be recognized that where the main mold frame (9) does notdirectly support the thin mold face (4), it does so indirectly bysupporting the foundation means (10) which in turn supports the thinmold face. The main mold frame may be provided with cooling channels(13) according to known methods. Injector pin bushing (12) and otherconventional aspects of an injection molding device are provided in themanner well known to those skilled in the art.

As illustrated in FIG. 1, according to the present invention, neitherall mold pieces of an injection mold device nor all portions of anysingle mold piece will necessarily have means for heating and/orvariable rate cooling.

The controlled cooling of the present invention provides a significantadvantage in that it is practical to heat the mold face in conjunctionwith such controlled cooling and yet achieve short molding cycle times.Heating the mold face provides significant advantage in that lowerinjection pressures may be used. Significant cost savings can berealized in the design and construction of mold devices for use withsuch lower injection pressures. Moreover, the use of lower injectionpressure can provide molded parts having improved part quality,specifically, less cracking, distortion, molecular orientation andbi-refringence.

It should be understood that the disclosure is for the purpose ofillustrations only and includes all modifications or improvements whichfall within the scope of appended claims.

We claim:
 1. A mold for making injection molded parts comprising:atleast one mold piece, which mold piece defines at least in part a moldcavity; means for injecting molding material into said mold cavity;means for heating at least a portion of the mold cavity surface of saidmold during the injection of said molding material to maintain thetemperature of said injected material at a sufficient level to preventthe molecules of said injected material from solidifying into a specificorientation; means for cooling at least a portion of the mold cavitysurface of said mold; and means for controlling the rate of cooling ofat least one portion of the mold cavity surface of said moldindependently from at least one other portion of the mold cavity surfaceof said mold so as to maintain selected properties of said injectedmaterial substantially uniform throughout said mold cavity, said meansfor controlling the rate of cooling including at least one heat pipewhich has housing means forming a sealed chamber, fluid contained withinsaid sealed chamber, wicking means within said sealed chamber fortransporting the liquid phase of said fluid, and control means forcontrolling the thermal conductance of said heat pipe.
 2. A moldaccording to claim 1 wherein said thermal conductance control meanscontrols the volume of liquid in said sealed chamber.
 3. A moldaccording to claim 1 wherein there is substantially no permanentdeformation of said mold cavity during the molding cycle at an injectionpressure sufficient to produce an injection molded part of substantiallyequal volume to the mold cavity.
 4. A mold according to claim 1 whereinsaid heating means controllably heats at least one portion of the moldcavity surface of said mold independently from at least one otherportion of the mold cavity surface of said mold.
 5. A mold according toclaim 4 wherein said mold cavity surface has a thin mold face and saidheating means comprises at least one electrical resistance heater withinsaid thin mold face.
 6. A mold according to claim 1 wherein at least onemold piece comprises:a thin mold face forming the cavity-side surfacethereof; a mold frame connected to and supporting, at least in part, theperimeter of said thin mold face; and a foundation means located betweensaid mold frame and said thin mold face for supporting said thin moldface.
 7. A mold according to claim 6 wherein said foundation meanscomprises a support structure which provides access to said thin moldface for said cooling rate control means.
 8. A mold according to claim 7wherein said heat pipe extends through said support means from said thinmold face to said mold frame.
 9. A mold according to claim 8 wherein achannel in the mold frame forms a portion of the heat pipe to operate asat least a part of the condenser thereof.
 10. A mold according to claim1 wherein said thermal conductance control means controls the volume ofsaid sealed chamber.
 11. A mold according to claim 10 wherein said meansfor controlling the volume of said sealed chamber comprises bellowmeans, the cavity of which bellow means is of controllably variablevolume and comprises a portion of said sealed chamber.
 12. A moldaccording to claim 1 wherein said thermal conductance control meanscomprises means for controlling the amount by weight of fluid in saidsealed chamber.
 13. A mold according to claim 12 wherein said controlmeans comprises a fluid reservoir and reversible pump means forming aconnection between said fluid reservior and said sealed chamber.
 14. Amold according to claim 13 wherein said reversible pump means comprisesa reversible metering pump.
 15. A mold according to claim 12 whereinsaid control means comprises at least one uni-directional valve.
 16. Adevice according to claim 15 wherein said uni-directional valvecomprises a metered valve.